TL;DR: MAKE SPELL LISTS FOR YOUR SETTING! π¨
They are great at tying mechanics and flavour together, while also giving you a greater understanding of the setting you are in. They require a bit of work though.
Long π§΅π
So, a short explanation for the uninitiated:
In D&D each player character has a class. Some classes have access to magic spells. Which spells are available to you is dictated by a class spell list.